Washington: hornbeam discharge

We have five hornbeam trees (approx.8 years old) that this year are discharging an invisible, very sticky substance to anything below (including our hot tub). One source indicates this could be the work of aphids, but the discharge seems so pervasive, and I've not observed any aphids on the trees. Any ideas on what is causing this?
Thanks,
Dante

We do have aphids!

 

How tall are the trees? Have you noticed any ants? Another thought I have is, is the ground around them overly wet? (I'm thinking a gummosis type of a discharge like cherry trees can get.)

 

Sounds very much like honeydew (i.e., aphid pee)

 

Thanks for the input. I do believe I have an aphid invasion. Am working with an arborist on this, but any "home remedies" will be appreciated.

 

ladybugs are effective for controlling aphids, not to mention fun!